If you're trying to conceive, you can find out how to give yourself the best chance by reading Trying to get pregnant.
Even if babies are the last thing on your mind at the moment, you can take steps to help maintain your fertility.
Professor William Ledger, Professor of Obstetrics and Gynaecology at the University of Sheffield, explains the basics.
How age affects your fertility
When it comes to fertility, age matters.
Many people today wait until they're older to have children. But fertility declines over time, and you should consider this if you plan to have children later.
Both women and men are at their most fertile in their early twenties.
In women, fertility declines more quickly with age. This decline becomes rapid after the age of 35, particularly due to the decline in the quality of the eggs released by the ovaries.
Around one-third of couples in which the woman is over 35 have fertility problems. This rises to two-thirds when the woman is over 40.
Men's fertility gradually declines from around the age of 40, but most men are able to father children into their 50s and beyond.
Avoid STIs to stay fertile
If you think you might have caught an STI, go to your GP or a sexual health clinic.
Find out more about STIs.
Stop smoking to protect your fertility
"Women who smoke 20 cigarettes a day experience the menopause on average two years earlier," says Professor Ledger. Men who smoke risk damaging their sperm.
Get advice on stopping smoking.
Stay a healthy weight if you're trying for a baby
Being underweight or overweight can lower your chances of conceiving.
One cause of infertility is polycystic ovary syndrome (PCOS), which is made worse by being overweight or obese.
Check whether you're overweight with our BMI Healthy weight calculator.
Lose weight in 12 weeks using this NHS-approved weight loss plan.
Avoid alcohol when trying to get pregnant
If you're pregnant, or planning to become pregnant, the safest approach is not to drink alcohol at all, to keep risks to your baby to a minimum.
Your partner should drink no more than 14 units of alcohol a week, which should be spread evenly over three days or more. Drinking too much alcohol can affect the quality of his sperm.
See Drinking and alcohol for information on low risk drinking.
Keep your testicles cool to boost your sperm count
A man's testicles should be one or two degrees cooler than the rest of his body. Tight underwear, hot showers and hot baths can all raise the temperature of the testicles and slow sperm production.
Read about the causes of low sperm count.
Avoid radiation to protect your fertility
Exposure to radiation and chemicals such as glycol ester, found in some paints, can damage fertility.
What to do if you are having problems conceiving
If you've been trying to conceive for a year or more (or six months if you're a woman over the age of 35) by having regular unprotected sex and are still not pregnant, it's time to see your GP.
Your GP can do tests to identify possible fertility problems, and can provide advice on the next steps.
Fertility tests for men and women.
Learn more in Support for problems getting pregnant.
Read more articles about fertility and conception.
Article provided by NHS Choices